Caffeine is a diuretic, but you'd actually have to drink about 5 litres of diet coke for it to make any noticable difference in your blood chemistry. and by that point, you'd be peeing for england, bloated, probably puking and be suffering palpitations due to the extreme caffeine.
aspartame is also a mild dehydrant, but again it's negligible if you're sticking to a pint or two a day, spread out and not in one go.
to reduce dehydrant issues, go for asda'sown brand cola zero. it's nice tasting and only contains the caffeine that's a problem, but not the aspartame.